“Mini-sub arriving in about 17 hours,” he wrote. “Hopefully useful. If not, perhaps it will be in a future situation.”

Rescue divers extracted four boys from the cave Sunday and headed back Monday to rescue the remaining eight and their soccer coach.

Thai officials estimated the rescue operation would take 10 hours and end by 9 p.m. on Monday. Early reports suggest it may progress even faster.

The four boys who were rescued Sunday had to travel about 2 1/2 miles with oxygen tanks. They made their way back to the cave’s entrance using a 3-mile rope. They were accompanied by two professional divers each throughout.